The VH4-34 immunoglobulin heavy chain genes, largely independent of the antibody D or J genes or the light chains, encode natural autoantibodies that recognize the i-antigen (polylactosamine [3Gal-beta-4GlcNAcbeta-]n) and/or I-antigen (GlcNAc-beta6-branched polylactosamine). The iI-antigen is a post-translational modification found on various glycoproteins and glycolipids in humans and on a number of pathogens. Neoplastic transformation and particular infections causing proliferations of VH4-34 + B cells can also cause pathological cold-agglutinin disease resulting in hemolytic anemia and the accumulation of antibody-erythrocyte complexes in small vessels. The VH4-34 gene has also been reported to play an increased role in various other autoimmune diseases including lupus, rheumatism, and multiple sclerosis. Despite the dangerous specificity encoded by VH4-34-utilizing antibodies, humans utilize VH4-34 to encode nearly 10% of their initial, antigen-naive repertoire. B cells utilizing VH4-34 are tightly regulated in T-dependent immune responses and are normally excluded from IgG antibody responses, further demonstrating the dangerous potential of this VH gene. It is important to define why humans have evolved to over-utilize the potentially pathological VH4-34 so profoundly in their early B cells only to exclude it from classic secondary immune responses and how these processes occur. In specific aim 1 the hypothesis is tested that the high prevalence of VH4-34 utilizing naive B cells in humans is due to an important role against a dangerous pathogen expressing glycans similar to human i/I antigens and that the protective immunity provided by this VH gone is more important than its dangerous potential. In specific aim 2, the molecular reason for VH4-34 over-representation in the naive B cell repertoire will be elucidated. In specific aim 3, a mouse model of VH4-34 immunity and tolerance will be generated and analyzed. The experiments proposed will benefit the specific disease processes associated with the VH4-34 gene and will provide many valuable insights into basic immunology topics including analysis of the delicate balance between immunity and autoimmunity that so often breaks down to cause devastating autoimmune diseases, control of V gene expression, immune evolution, and immunity to glycan antigens.